JP2020136683A - Semiconductor device and manufacturing method thereof - Google Patents

Abstract

To provide a semiconductor device having an N-polar AlN layer having high surface flatness.SOLUTION: A semiconductor device 10 includes a sapphire substrate 11 having an off angle of 0.5° or more and 5.0° or less with respect to a c-plane of the main surface, and an N-polar AlN layer 12 formed by epitaxially growing an N-polar AlN on top of the sapphire substrate. The surface roughness (RMS) of the N-polar AlN layer 12 is 0.85 nm or less.SELECTED DRAWING: Figure 1

Description

The present invention relates to a semiconductor device and a method for manufacturing the same.

A technique is known in which N-polar GaN is epitaxially grown on a substrate whose main surface has an off angle with respect to the c-plane. For example, Patent Document 1 discloses that the surface of a sapphire substrate having an off angle of 0.5 ° or more and 2.0 ° or less with respect to the c-plane is nitrided, and N-polar GaN is epitaxially grown on the surface. Further, in Non-Patent Document 1, N-polar GaN, N-polar AlGaN, and N-polar GaN are described on sapphire substrates having off angles of 0.2 °, 0.8 °, and 2.0 ° with respect to the c-plane. High Electron Mobility Transistors (HEMTs) are disclosed, which are manufactured by epitaxially growing the transistors in order.

Japanese Unexamined Patent Publication No. 2009-147271

K. Prasertsuk et al., Appl. Phys. Express 11, 015503 (2018)

By the way, in some power devices and high frequency devices, two dimensional electron gas (2DEG) induced in heterojunction of a semiconductor is used as a channel. Then, the improvement of the energy conversion efficiency of these semiconductor devices can be achieved by increasing the concentration of the two-dimensional electron gas. However, in the heterojunction of the AlGaN / GaN structure in which AlGaN is laminated on GaN that is currently in practical use, it cannot be expected that the concentration of the two-dimensional electron gas will be higher than the current concentration. Therefore, as a new heterojunction of semiconductors, a structure in which a semiconductor such as GaN is laminated on AlN is conceivable, and by applying N-polar AlN to this, it is expected that the concentration of two-dimensional electron gas will be increased. it can. However, in order to form a heterojunction that induces a high concentration of two-dimensional electron gas, it is necessary that the surface flatness of AlN is high.

An object of the present invention is to provide a semiconductor device provided with an N-polar AlN layer having a highly flat surface.

In the present invention, a base substrate having an off angle of 0.5 ° or more and 5.0 ° or less with respect to the c-plane of the main surface and an N-polar AlN layer formed by epitaxially growing N-polar AlN on the base substrate are provided. It is a provided semiconductor device.

The present invention comprises a step of epitaxially growing N-polar AlN on a base substrate having an off-angle of the main surface with respect to the c-plane of 0.5 ° or more and 5.0 ° or less to form an N-polar AlN layer. It is a manufacturing method.

According to the present invention, N-polarity AlN is epitaxially grown on the main surface of the base substrate whose off-angle with respect to the c-plane is 0.5 ° or more and 5.0 ° or less, so that the surface is highly flat. An AlN layer can be obtained.

It is sectional drawing of the high electron mobility transistor which concerns on embodiment. It is explanatory drawing which shows the 3rd step of the manufacturing method of the high electron mobility transistor which concerns on embodiment. It is explanatory drawing which shows the 4th step of the manufacturing method of the high electron mobility transistor which concerns on embodiment. It is explanatory drawing which shows the 5th step of the manufacturing method of the high electron mobility transistor which concerns on embodiment. It is a timing chart when AlN was crystal-grown on the sapphire substrate in Test Evaluation 1 of the Example. It is a graph which shows the relationship between the off angle with respect to the c-plane of the main surface of a sapphire substrate, and the surface roughness (RMS) of the surface of an N polar AlN layer. It is a 1st timing chart of hydrogen etching in the test evaluation 2 of an Example. It is a 2nd timing chart of hydrogen etching in the test evaluation 2 of an Example. It is a graph which shows the relationship between the hydrogen etching time and the surface roughness (RMS) of the surface of an N polar AlN layer. It is a timing chart of the crystal growth of N polar AlN and hydrogen etching in the test evaluation 3 of an Example. It is a graph which shows the relationship between the crystal growth time of N polar AlN per one time and the surface roughness (RMS) of the surface of an N polar AlN layer.

Hereinafter, embodiments will be described in detail with reference to the drawings.

FIG. 1 shows a high electron mobility transistor 10 (semiconductor device) according to an embodiment. The high electron mobility transistor 10 is a field effect transistor having a channel of two-dimensional electron gas (2DEG) induced in a heterojunction of a semiconductor.

The high electron mobility transistor 10 according to the embodiment includes a sapphire substrate 11 as a base substrate, an N-polar AlN layer 12 provided on the sapphire substrate 11, and an N-polar semiconductor layer provided on the N-polar AlN layer 12. An N-polar GaN layer 13 and a gate electrode 14, a source electrode 15, and a drain electrode 16 provided on the N-polar GaN layer 13, respectively, are provided. Here, the term "N-polarity" in the present application means that the surface of the semiconductor crystal growth side orthogonal to the thickness direction of each layer is inclined from the N-polar plane (-c plane, N-plane) or the N-polar plane. Moreover, it means that it is a semi-polar surface mainly including an N-polar surface.

The off-angle of the main surface of the sapphire substrate 11 with respect to the c-plane is 0.5 ° or more and 5.0 ° or less. That is, the main surface of the sapphire substrate 11 is a surface whose c-plane is inclined by 0.5 ° or more and 5.0 ° or less with the a-axis or m-axis as the center. The off angle of the main surface of the sapphire substrate 11 with respect to the c-plane is preferably 1.5 ° or more, more preferably 1.5 ° or more, from the viewpoint of obtaining the N-polar AlN layer 12 having a high surface flatness as described later. Is also large, more preferably 2.0 ° or more, preferably 4.5 ° or less, and more preferably 4.0 ° or less. Here, the "main surface" in the present application is a surface perpendicular to the direction of semiconductor stacking growth, and is usually the widest surface on the surface.

The N-polar AlN layer 12 is formed by epitaxially growing N-polar AlN on the main surface of the sapphire substrate 11. According to the high electron mobility transistor 10 according to the embodiment, N-polar AlN is epitaxially grown on the main surface of the sapphire substrate 11 whose off angle with respect to the c-plane is 0.5 ° or more and 5.0 ° or less. As a result, an N-polar AlN layer 12 having a high surface flatness can be obtained. The N-polar AlN layer 12 may be doped with a dopant, but it is preferably undoped because it is desirable that the impurities are as low as possible. The thickness of the N-polar AlN layer 12 is, for example, 100 nm or more and 5000 nm or less.

The N-polar GaN layer 13 is formed by epitaxially growing N-polar GaN of an N-polar semiconductor on the surface of the N-polar AlN layer 12, and also forms a heterojunction structure with the N-polar AlN layer 12. .. Due to the heterojunction structure of the N-polar AlN layer 12 and the N-polar GaN layer 13, a large band offset occurs on their joint surfaces. Therefore, a layered two-dimensional electron gas (2DEG) induced by this heterojunction is formed in the vicinity of the junction surface of the N-polar GaN layer 13 with the N-polar AlN layer 12. Therefore, the N-polar GaN layer 13 constitutes a channel layer, and a large current and high electron mobility can be obtained in this two-dimensional electron gas (2DEG). Further, the high band offset at the junction surface of the heterojunction structure of the N-polar AlN layer 12 and the N-polar GaN layer 13 makes it possible to increase the concentration of the two-dimensional electron gas (2DEG) and improve the energy conversion efficiency. From that point of view, the concentration of the two-dimensional electron gas (2DEG) is preferably higher than 1.0 × 10 ¹³ / cm ² , more preferably 3.0 × 10 ¹³ / cm ² or more, still more preferably 5.0. × 10 ¹³ / cm ² or more. The electron mobility in the N-polar GaN layer 13 is greater than 2000 cm ² / Vs.

The crystal growth surface of the N-polar GaN layer 13 is preferably a semi-polar surface from the viewpoint of obtaining the N-polar GaN layer 13 having good crystal quality. The thickness of the N-polar GaN layer 13 is, for example, 100 nm or more and 5000 nm or less.

A pair of recesses 13a reaching the two-dimensional electron gas (2DEG) are formed in the N-polar GaN layer 13 at intervals. The gate electrode 14 is provided on the surface of the N-polar GaN layer 13 between the pair of recesses 13a. The gate electrode 14 is composed of, for example, a laminated film of a Ni film and an Au film. The source electrode 15 and the drain electrode 16 each fill a pair of recesses 13a formed in the N-polar GaN layer 13 so as to come into contact with the two-dimensional electron gas (2DEG) and project from the surface of the N-polar GaN layer 13. It is provided. The source electrode 15 and the drain electrode 16 are composed of, for example, a laminated film of a Ti film and an Al film.

Next, a method of manufacturing the high electron mobility transistor 10 according to the embodiment by using the metalorganic vapor phase growth method (MOVPE) will be described.

In the method for manufacturing the high electron mobility transistor 10 according to the embodiment, first, in the first step, the sapphire substrate 11 is heated to a predetermined AlN crystal growth temperature in a high-pressure atmosphere in which a carrier gas is circulated, and then the temperature is raised to a predetermined AlN crystal growth temperature. The pressure is reduced while maintaining the temperature. The AlN crystal growth temperature is preferably 1200 ° C. or higher and 1450 ° C. or lower, more preferably 1300 ° C. or higher and 1400 ° C. or lower, from the viewpoint of obtaining N-polar AlN having high crystal quality.

Next, in the second step, the main surface of the sapphire substrate 11 is subjected to nitriding treatment by flowing an N source gas such as NH ₃ gas together with the carrier gas and bringing it into contact with the sapphire substrate 11.

Subsequently, in the third step, the Al source gas and the N source gas are circulated together with the carrier gas and brought into contact with the sapphire substrate 11, so that the N polar AlN is placed on the main surface of the sapphire substrate 11 as shown in FIG. 2A. Is epitaxially grown to form an N-polar AlN layer 12. Examples of the Al source include trimethylaluminum (TMA) and triethylaluminum (TEA). Examples of the N source include NH _{3 and the} like.

At this time, since the sapphire substrate 11 having an off angle of the main surface with respect to the c surface of 0.5 ° or more and 5.0 ° or less is used, an N-polar AlN layer 12 having a high surface flatness can be obtained. The surface roughness (RMS) of the surface of the N-polar AlN layer 12 is preferably 5 nm or less, more preferably 3 nm or less, and further preferably 1 nm or less. This surface roughness (RMS) is measured by atomic force microscopy.

Further, from the viewpoint of improving the flatness of the surface of the N-polar AlN layer 12, it is preferable to hydrogen-etch the step bunching on the surface of the N-polar AlN at this time. This hydrogen etching may be performed only once after the formation of the N-polar AlN layer 12. In this case, the hydrogen etching time is, for example, 30 seconds or more and 60 minutes or less, preferably 5 minutes or less, and more preferably 3 minutes or less. From the viewpoint of improving the flatness of the surface of the N-polar AlN layer 12, in the crystal growth process of the N-polar AlN in the third step, the crystal growth of the N-polar AlN and the hydrogen etching of the surface of the N-polar AlN are alternately performed. It is preferable to repeat. In this case, the crystal growth time of N-polar AlN at one time is preferably 15 minutes or less, more preferably 10 minutes or less, and further preferably 5 minutes or less. The hydrogen etching time per operation is, for example, 30 seconds or more and 60 minutes or less, preferably 5 minutes or less, and more preferably 3 minutes or less.

Hydrogen etching on the surface of N-polar AlN is preferably performed in a pulsed manner. “Hydrogen etching is performed in a pulsed manner” means that the surface of N-polar AlN is periodically etched with H ₂ gas. This is possible, for example, by arranging the surface of the N-polar AlN in an atmosphere in which the H ₂ gas and the NH ₃ gas flow, and periodically switching the flow of the NH ₃ gas on / off. When the NH ₃ gas flow is off, only the H ₂ gas flows and the hydrogen etching on the surface of the N-polar AlN is turned on. On the other hand, when the flow of NH ₃ gas is on, H ₂ gas and NH ₃ gas flow, and hydrogen etching on the surface of N-polar AlN is turned off by the action of NH ₃ gas. The on and off times of hydrogen etching are, for example, 1 second or more and 2 minutes or less, respectively. The on-time and off-time of hydrogen etching may be the same or different.

The temperature condition for hydrogen etching is, for example, 1150 ° C. or higher and 1500 ° C. or lower. The flow rate of H ₂ gas is, for example 100sccm least 20SLM less. The pressure condition for hydrogen etching is, for example, 2 kPa or more and 50 kPa or less. The total time of hydrogen etching is, for example, 30 seconds or more and 60 minutes or less.

The surface roughness (RMS) of the surface of the N-polar AlN layer 12 obtained by hydrogen etching is preferably 0.85 nm or less, more preferably 0.75 nm or less, still more preferably 0.7 nm or less, still more preferably. Is 0.65 nm or less, more preferably 0.5 nm or less, and if various conditions are optimized, a 0.3 nm level can be expected.

Subsequently, in the fourth step, the Ga source gas and the N source gas are circulated together with the carrier gas and brought into contact with the N-polar AlN layer 12 on the sapphire substrate 11, so that the N-polar AlN layer is brought into contact with the N-polar AlN layer 12 as shown in FIG. 2B. N-polar GaN is epitaxially grown on the surface of 12 to form the N-polar GaN layer 13. Examples of the Ga source gas include TMG and TEG. Examples of the N source include NH _{3 and the} like. The crystal growth conditions are appropriately set corresponding to the N-polar GaN forming the N-polar GaN layer 13.

Finally, in the fifth step, as shown in FIG. 2C, a recess 13a is formed in the N-polar GaN layer 13 by etching using a photolithography method, and then the gate electrode 14, the source electrode 15, and the drain electrode 16 are formed. Form a film.

In the above embodiment, the high electron mobility transistor 10 is shown as the semiconductor device, but the present invention is not particularly limited thereto, and the semiconductor light emitting device (LED), the laser diode (LD), the solar cell, and the field effect are used. It may be a transistor (FET), a light receiving element, or the like.

Further, in the above embodiment, the base substrate is the sapphire substrate 11, but the present invention is not particularly limited to this, and for example, a SiC substrate or the like may be used.

Further, in the above embodiment, the N-polar semiconductor layer on the N-polar AlN layer 12 is the N-polar GaN layer 13, but the present invention is not particularly limited to this, and other components such as the N-polar AlGaN layer and the N-polar AlGaInN layer are used. It may be an N-polar III nitride layer of.

Further, in the above embodiment, the production method using the organic metal vapor phase growth method (MOVPE) has been shown, but the present invention is not particularly limited to this, and the hydride vapor phase growth method (HVPE) and the plasma vapor phase growth method are used. (PECVD), molecular beam crystal growth method (MBE), chemical vapor deposition method (CVD) such as hot gas phase growth method; physical vapor phase method (PVD) such as spatter rig method, vapor deposition method, etc. You may.

[Test evaluation 1]
Substrate sapphire with off-angles of the main surface relative to the c-plane of 0.2 °, 1.0 °, 1.5 °, 2.0 °, 3.0 °, 4.0 °, and 5.0 ° A plurality of boards were prepared for each. Then, N-polar AlN was epitaxially grown on each sapphire substrate by the MOVPE method to form an N-polar AlN layer.

Specifically, as shown in FIG. 3, (i) First, while introducing _{H 2} gas, under pressure 100 kPa, and the temperature of the sapphire substrate was heated to 1300 ° C. over 16 minutes. (Ii) When the temperature of the sapphire substrate reached 1300 ° C., the pressure was reduced from 100 kPa to 20 kPa over 10 minutes. (Iii) then, it begins to conduct NH ₃ gas, after flowing for 1 minute NH ₃ gas was stopped after flowing (iv) begins to conduct TMA gas, 30 seconds TMA gas. (V) subsequently, after flowing continuously only 10 seconds _{H 2} gas and NH ₃ gas, it begins to conduct (vi) again TMA gas was flowed for 30 minutes TMA gas. At this time, the TMA gas and the NH ₃ gas are such that the V / III ratio, which is the ratio of the number of moles supplied of the NH ₃ gas, which is the N source gas, to the number of moles supplied by the TMA gas, which is the Al source gas, is 5. The supply flow rate of was 29.2 sccm and 2.9 sccm, respectively. (Vii) Then, after 30 minutes, the TMA gas was stopped, and at the same time, the H ₂ gas and the NH ₃ gas were also stopped, and instead, the N ₂ gas was started to flow and cooled to room temperature as it was. The surface roughness (RMS) of the surface of the N-polar AlN layer formed on each sapphire substrate was measured by atomic force microscopy.

FIG. 4 shows the relationship between the off-angle of the main surface of the sapphire substrate with respect to the c-plane and the surface roughness (RMS) of the surface of the N-polar AlN layer. According to FIG. 4, when the off angle of the main surface of the sapphire substrate with respect to the c surface is 0.5 ° to 5.0 °, the surface of the N-polar AlN layer is compared with the case where the off angle is 0.2 °. It can be seen that the surface roughness (RMS) is small and therefore its flatness is high.

[Test evaluation 2]
Prepare a plurality of sapphire substrates of the base substrate whose off angle with respect to the c-plane of the main surface is 2.0 °, and perform the same operation as in Test Evaluation 1 up to each (vi) step, and perform the same operation as in Test Evaluation 1 to form an N-polar AlN layer on the sapphire substrate. Was formed. Then, as shown in FIGS. 5A and 5B, the step bunching on the surface of the N-polar AlN was hydrogen-etched by stopping the TMA gas and the NH ₃ gas and allowing only the H ₂ gas to flow for 10 seconds, followed by the H ₂ gas. The process of flowing NH ₃ gas for 10 seconds to stop hydrogen etching, stopping NH ₃ gas again and flowing only H ₂ gas for 10 seconds to hydrogen etch the surface of N-polar AlN, with a hydrogen etching time of 2 minutes. I went until it became. That is, hydrogen etching was performed in a pulsed manner. Then, (vii) the process proceeds to step stops the H ₂ gas, instead begins to conduct N ₂ gas and allowed to cool to room temperature. Further, the same experiment was performed when the hydrogen etching time was set to 30 minutes and 45 minutes, respectively. The V / III ratio and the supply flow rates of TMA gas and NH ₃ gas were the same as in Test Evaluation 1.

The surface roughness (RMS) of the surface of the final N-polar AlN layer was measured by atomic force microscopy in each case where the hydrogen etching time was set to 2, 30 minutes, and 45 minutes, and the average value was measured. Calculated. The surface roughness (RMS) of the surface of the N-polar AlN layer was 0.81 nm when the hydrogen etching time was 2 minutes, 0.62 nm when the hydrogen etching time was 30 minutes, and 0.65 nm when the hydrogen etching time was 45 minutes. When hydrogen etching was not performed, the value was 1.8 nm from Test Evaluation 1.

FIG. 6 shows the relationship between the hydrogen etching time and the surface roughness (RMS) of the surface of the N-polar AlN layer. According to FIG. 6, it can be seen that the flatness of the surface of the N-polar AlN layer is enhanced by performing hydrogen etching. Further, even if hydrogen etching is relatively short for about 2 minutes, a large surface flattening effect can be obtained, and even if the hydrogen etching time is longer than that, a slight increase in the effect is observed, but the effect is leap. It can be seen that there is no increase in the target.

In the N-polar AlN crystal grown on the sapphire substrate of the base substrate whose off angle with respect to the c-plane of the main surface is 0.2 °, large flocs are generated on the surface, and even if they are hydrogen-etched, they are small. Crystal grain boundaries appeared, and the above-mentioned surface flattening effect could not be obtained. That is, unless the off angle of the main surface of the base substrate with respect to the c surface is large to some extent, the effect of surface flattening by hydrogen etching cannot be obtained.

[Test evaluation 3]
Prepare a plurality of sapphire substrates as a base substrate having an off angle of 2.0 ° with respect to the c-plane of the main surface, perform the same operations as in Test Evaluation 1 up to step (v), and then perform the same operations as in Test Evaluation 1, as shown in FIG. , The operation of crystal growth of N-polar AlN under the same conditions as step (vi) of test evaluation 1 for 15 minutes, and then hydrogen etching under the same conditions as test evaluation 2 for 2 minutes was repeated twice for a total time. Crystal growth of N-polar AlN was carried out for 30 minutes. Then, (vii) the process proceeds to step stops the H ₂ gas, instead begins to conduct N ₂ gas and allowed to cool to room temperature. In addition, an experiment in which the total time for crystal growth of N-polar AlN was 30 minutes, crystal growth of N-polar AlN for 6 minutes and hydrogen etching for 2 minutes was repeated 5 times, and crystal growth of N-polar AlN for 3 minutes and 2 minutes. An experiment was also conducted in which the hydrogen etching of the above was repeated 10 times. The V / III ratio and the supply flow rates of TMA gas and NH ₃ gas were the same as in Test Evaluation 1.

The surface roughness (RMS) of the final surface of the N-polar AlN layer was measured by an atomic force microscope in each case where the number of repetitions of crystal growth and hydrogen etching of N-polar AlN was 2, 5, and 10 times. It was measured by the method and the average value was calculated. The surface roughness (RMS) of the surface of the N-polar AlN layer was 0.81 nm when the number of repetitions was 2 times, 0.73 nm when the number of repetitions was 5, and 0.68 nm when the number of repetitions was 10. When the crystal growth of N-polar AlN for 30 minutes and the hydrogen etching for 2 minutes were performed once, the value was 0.81 nm from Test Evaluation 2.

FIG. 8 shows the relationship between the crystal growth time of N-polar AlN and the surface roughness (RMS) of the surface of the N-polar AlN layer. According to FIG. 8, in the crystal growth process of N-polar AlN, the flatness of the surface of the N-polar AlN layer is enhanced by alternately repeating the crystal growth of N-polar AlN and the hydrogen etching on the surface of N-polar AlN. It turns out that it can be done. Further, it can be seen that the effect of surface flattening by hydrogen etching is enhanced by shortening the crystal growth time of N-polar AlN per time and performing hydrogen etching in small steps.

The present invention is useful in the technical field of semiconductor devices and methods of manufacturing them.

10 High electron mobility transistor (semiconductor device)
11 Sapphire substrate (base substrate)
12 N-polar AlN layer 13 N-polar GaN layer (N-polar semiconductor layer)
13a Recess 14 Gate electrode 15 Source electrode 16 Drain electrode

Claims (9)

The base substrate whose off angle with respect to the c-plane of the main surface is 0.5 ° or more and 5.0 ° or less,
An N-polar AlN layer formed by epitaxially growing N-polar AlN on the base substrate,
Semiconductor device equipped with. In the semiconductor device according to claim 1,
A semiconductor device having a surface roughness (RMS) of 0.85 nm or less of the N-polar AlN layer. In the semiconductor device according to claim 1 or 2.
A semiconductor device in which the off-angle of the main surface of the base substrate with respect to the c-plane is larger than 1.5 °. In the semiconductor device according to any one of claims 1 to 3.
A semiconductor device in which the base substrate is a sapphire substrate. In the semiconductor device according to any one of claims 1 to 4.
A semiconductor device in which an N-polar semiconductor is formed by epitaxially growing on the N-polar AlN layer and further provided with an N-polar semiconductor layer forming a heterojunction structure with the N-polar AlN layer. In the semiconductor device according to claim 5,
A semiconductor device in which the N-polar semiconductor layer is an N-polar GaN layer. N-polar AlN is epitaxially grown on a base substrate having an off angle of the main surface with respect to the c-plane of 0.5 ° or more and 5.0 ° or less to form an N-polar AlN layer. At this time, the surface of the N-polar AlN is formed. A method for manufacturing a semiconductor device, which comprises a step of hydrogen etching. In the method for manufacturing a semiconductor device according to claim 7,
A method for manufacturing a semiconductor device in which hydrogen etching on the surface of the N-polar AlN is performed in a pulsed manner. In the method for manufacturing a semiconductor device according to claim 7 or 8.
A method for manufacturing a semiconductor device in which crystal growth of the N-polar AlN and hydrogen etching of the surface of the N-polar AlN are alternately repeated in the crystal growth process of the N-polar AlN.

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